The conventional distributor for an internal combustion engine is shown in FIG. 1. A distributor housing 10 mounted in an internal combustion engine (not shown) consists of two portions, namely an upper distributor housing portion 11 and a lower shaft-bearing portion 12. The lower portion 12 has a cylindrical shape and has an axial bore 13 in which a rotary shaft 14 is inserted and supported at the upper end by a bearing member 15 and at the lower end by a metal sleeve 16. The rotary shaft 14 at the upper end is integrally connected with a bushing 17 upwardly extending axially of an enlarged space 18 in the upper portion 11 of the housing 10. The upper portion 11 at its upper end is engaged with and closed by a cap 19 secured to the housing 10 by screws 20. The bushing 17 has a rotary disc 21 secured vertically divided between upper and lower parts 17a, 17b fixed by a screw 22.
The rotary disc 21 extends radially and outwardly of the bushing 17. The bushing 17 at its top end is connected with a distributor rotor 23 which is secured by inserting the upper end of the bushing 17 into a bore provided in the distributor rotor 23 and fixed by a screw 24. The distributor rotor 23 is made of synthetic resin and at its upper surface has a rotor electrode 25. The rotor electrode 25 at one end is positioned about the rotating axis of the rotor 23 and is slidably connected with a contact 26 provided at the lower end of a central electrode 27, which is arranged in alignment of the rotary axis of the distributor rotor 23 and secured in a bore provided in the top wall of the cap 19. The electrode 25 at its other and outer end moves and passes near the side wall of the cap 19 and, successively, a plurality of circumferential electrodes 28 with a discharge gap. The circumferential electrodes 28 are positioned about the central electrode 27 circumferentially of the top wall of the cap 19 and spaced one to the other with a predetermined angular interval.
The shaft 14 is rotated synchronously with the revolution of the internal combustion engine and rotates the disc 21 integrally therewith. The disc 21 has a radial slit at a position at which it faces to a photoelectric pickup. The photoelectric pickup consists of a light-emitting element 29 and a light-receiving element 30 which are arranged opposite to each other with respect to a symmetric plane containing the rotary disc 21. The unit 31 including the light-emitting and light-receiving elements 29, 30 has also a signal processing circuit 32 for processing output signal from the photoelectric pickup. 31a is an element that acts to protect the unit from radio frequency interference (RFI).
In the above-mentioned apparatus, the shaft 14 rotates synchronously with the revolution of the engine (not shown). In the rotation of the shaft 14, the slit in the rotary disc 21 crosses an infrared light beam and intermittently passes it between the light-emitting element 29 and the light-receiving element 30. The corresponding output signal from the light-receiving element 30 is processed for its wave-form shaping and the like by signal processing circuit 32, thereby to obtain a predetermined crank angle signal or cylinder discriminating reference signal. Such a signal determines ignition time when the ignition high voltage is supplied to the central electrode 27 from where it is successively distributed to the plurality of circumferential electrodes 28 through the contact 26 and the rotor electrode 25 so that ignition is provided in each cylinder of the internal combustion engine (not shown).
In a conventional distributor as described above, however, electrical noise from outside is induced into the signal processing circuit 32. This may cause operational errors, in particular erroneous ignition and missing ignition of the internal combustion engine.
As the result of careful investigation on this problem, the following cause therefor has been found. When the center electrode 27 is supplied with ignition high voltage, this voltage is induced to the distributor rotor 23, the bushing 17 and the shaft 14. Usually the shaft 14 is connected with the housing 10 for grounding thereof. However, at the motion transmitting portion at the bearing 15 and between the shaft 14 and the sleeve metal 16, there can exist some clearance or gap which may cause the the shaft to lose contact with its electrical ground, i.e., to be temporarily ungrounded during high acceleration or deceleration of the engine. A high voltage which is induced to the bushing 17, the shaft 14, etc., in this state is also induced to the signal processing circuit 32 and may cause errors in its operation.